The present invention relates to an apparatus and process for generating a spatial magnetic field having extremely high homogeneity.
FIG. 1 is a flow chart for determining an optimum output of a conventional correcting apparatus which acts on a crude magnetic field, generating a highly homogeneous magnetic field, as disclosed, for example, in the IBM Technical Disclosure Bulletin (Vol. 19, No. 9, 1977, pp. 3517-3519). In this example, a shim coil is used as the magnetic field correcting apparatus. The shim coil is capable of independently generating an output that corresponds to a given inhomogeneous component of the crude magnetic field. By installing many shim coils of different kinds or having different orders of magnetic field magnitude, it is possible to remove many inhomogeneities in the crude magnetic field.
In FIG. 1, reference numeral 1 denotes an arithmetic unit, 2 denotes optimization of a primary correction current driven by the arithmetic unit 1, 3 denotes optimization of a secondary correction current, reference numerals 4 and 5 denote feedback lines to the arithmetic unit 1 and the optimization of primary correction current, 6 denotes the completion of optimization by the shim coil, and 7 denotes NMR (nuclear magnetic resonance) spectrometers. The NMR spectrometer 7 subjects an FID (free induction decay) signal pulse received by a pulse NMR method to the Fourier transformation, and measures the homogeniety of the magnetic field in the measured sample by relying upon a half-value width of the signal pulse which has been Fourier-transformed. When the homogeneity of the magnetic field is poor, the FID signal pulse attenuates quickly. When the homogeneity of the magnetic field is extremely poor, it becomes difficult to carry out the measurement.
In the region where the correction output is generated, a correction magnetic field (not shown) has been generated already to correct and highly homogenize the crude magnetic field distribution.
First, the step of correcting the magnetic field is initiated by inputting a suitable start instruction to the arithmetic unit 1. That is, in a group of shim coils that generate primary magnetic field outputs, a particular shim coil is set at a given current value by the arithmetic unit 1, to initiate the optimization of the primary correction current as denoted at 2. Under the condition where the sim coil is set at a given current value and is generating a correcting magnetic field, the NMR spectrometer 7 measures the magnetic field homogeneity and compares it with the homogeneity under the previous condition where the correcting magnetic field is not generated by the shim coil. In this case, if the homogeneity is deteriorating, the intensity of electric current flowing through the shim coil is changed by the arithmetic unit 1 to change the correction magnetic field intensity generated by the shim coil. Then, the homogeneity of the magnetic field is measured and compared by the NMR spectrometer 7 in the same manner as above. A further possibility for correction is that both the direction of current flowing through the shim coil and the intensity thereof may be changed in order to change both the intensity of the correcting magnetic field generated by the shim coil and, again, the direction thereof, and the homogeneity of the magnetic field is measured and compared by the spectrometer 7 as above. The direction of current flowing in the shim coil is properly set when it becomes obvious through iterative measurement by the NMR spectrometer 7 that the homogeneity of the magnetic field is improved compared with previous tests when no current was supplied to the shim coil, after having repeated the trial and error many times. Next, the current flowing into the shim coil must be optimized. The object of this optimization is to adjust the correcting magnetic field generated by the shim coil until the distribution of the crude magnetic field no longer contains a magnetic field component which is the same as the correcting magnetic field component. Originally the homogeneity of the magnetic field is .alpha. where a current I is flowing into the shim coil. The current I flowing into the shim coil is then set at I+I by the arithmetic unit 1, and a homogeneity + of the generated magnetic field is measured by the NMR spectrometer 7. The current has now been changed by an amount .DELTA.I toward the direction to increase the homogeneity of the magnetic field provided .alpha.+.DELTA..alpha.&lt;.alpha.. Therefore, the current flowing through the shim coil is changed until (d.alpha.)/(dI)=0 is obtained.
Thus, the optimization of a shim coil is completed among the primary shim coils. Even of the remaining shim coils is optimized among the group of primary shim coils by the same method as mentioned above. As the optimization of all primary shim coils is completed, the optimization of the primary correction current is completed as denoted by 2. However, when (d.alpha.)/(dI)=0 does not hold true, it means that the optimization is not possible. Therefore, a signal is sent to the arithmetic unit 1 via the feedback line 4 to inform that the correction current is not optimized, whereby the step of correction is halted or the program proceeds to the next step though the optimum correction has not been accomplished.
After the primary correction current is optimized as described above, the secondary correction current is optimized as denoted by 3 by the NMR spectrometer 7 which repetitively measures the homogeneity in the magnetic field, and whereby the optimization is completed as denoted by 6.
Further, in order for the FID signal to be of sufficient intensity to be observed, the crude magnetic field must have a minimum homogeneity before it is corrected by the shim coils. When the measuring region within a crude field distribution is wide, the magnetic field usually varies in large amounts and the homegeneity in the magnetic field decreases as compared with that of the same field in a smaller region. Therefore, when wide regions are to be corrected and highly homegenized, the magnetic field homegeneity must be higher than that of the minimum measurable homogeneity of a smaller region. Otherwise, it becomes impossible to measure the homogeneity in the magnetic field by FID.
There are two types of shim coils, i.e., superconductive shim coils and ordinary conductive shim coils. The superconductive shim coils are used when the apparatus for generating crude magnetic field consists of a superconductive coil, and are installed in expensive liquid helium. To adjust the magnetic field generated by the shim coils, it is necessary to supply an electric current at all times to heat the heater of a heater-equipped switch which is called the permanent switch and which is also installed in the liquid helium. When the adjustment is effected over extended periods of time, therefore, the liquid helium evaporates in considerable amounts.
The ordinary conductive shim coils are used at ordinary temperatures and are obtained by winding an ordinary electric wire such as copper wire or aluminum wire.
According to the conventional apparatus for generating a highly homogeneous magnetic field constructed as described above, the magnetic field is corrected and homogenized on a trial-and-error basis. As the number of shim coils increases, therefore, the time required for the correction increases strikingly making it difficult to quickly obtain a highly homogeneous magnetic field. When the superconductive shim coils are used, furthermore, expensive liquid helium is consumed in tremendous amounts making the adjustment operation extremely costly. When the homogeneity in the crude magnetic field is poor, furthermore, the FID signal pulses are attenuated very quickly to make it difficult to highly homogenize the magnetic field.